Reliable and accurate earthwork calculation is one of the most important components in road engineering that can influence the construction cost and road alignment choice. In this research, the method of ground penetrating radar (GPR) was applied to detect the subsoil rock share in forest road construction. The GPR data acquisition was carried out using a 250 MHz GPR antenna along 22 segments on the centerline and 12 meters across to the road project. The GPR data processing was done by performing different filters, such as background removal, migration, band pass filter, and horizontal and vertical smoothing on the GPR data. To investigate the capability of the GPR method used in this road construction project, we compared the GPR radargram and road profile after construction. The results indicated that the maximum penetration depth of the GPR waves produced by a 250 MHz GPR transmitter antenna, which were between 2 and 5 meters in fine area (clayey soil with high moisture content) and rocky grounds, respectively. Moreover, the capability of GPR to map large dimension roots and water content is presented in this work. Comparison of the GPR records with direct observations of road profiles revealed that the GPR technique can yield a wide range of information that leads to a more efficient and effective earthwork operation of forest roads, although in some parts of the road profile the expected results from the target depth of rock share were not achieved.
